1. Field
The disclosed technology relates to voltage regulation, and in particular, to direct current (DC) voltage regulation.
2. Background
Voltage regulators for producing a regulated DC output voltage from an unregulated DC input voltage supply are known in the prior art. One such prior art voltage regulator which produces a regulated DC output voltage from an unregulated DC voltage supply is referred to as a linear voltage regulator. Such a prior art linear voltage regulator is illustrated in FIG. 1. The linear voltage regulator of FIG. 1 comprises a pair of input terminals 1 and 2 across which the unregulated DC voltage supply is applied, and a pair of output terminals 3 and 4 across which the regulated DC output voltage is produced and applied to a load 5. The input terminal 2 and the output terminal 4 are tied to a common voltage, typically ground voltage. The load current Il is drawn from the unregulated DC voltage supply and flows from the input terminal 1 to the output terminal 3 through a pass element 7 of variable impedance, which typically is provided by a field effect transistor M1.
A resistor-divider circuit 9 connected across the output terminals 3 and 4 produces a voltage on an intermediate tap 10 which is indicative of the output voltage produced across the output terminals 3 and 4. The voltage on the intermediate tap 10 is applied to one of an inverting input and a non-inverting input of an error amplifier 12, and a voltage reference Vref is applied to the other of the inverting input and the non-inverting input of the error amplifier 12 in order to produce a negative feedback loop with the pass element 7. In the voltage regulating circuit of FIG. 1 the voltage on the intermediate tap 10 is applied to the non-inverting input, and the voltage reference Vref is applied to the inverting input. The voltage reference Vref is of value substantially similar to the value of the voltage which should appear on the intermediate tap 10 when the output voltage produced across the output terminals 3 and 4 is at the correct regulated voltage value. The error amplifier 12 produces a control signal which is indicative of the difference between the voltage on the intermediate tap 10 and the voltage reference Vref. The control signal is applied to the gate of the field effect transistor M1. The impedance of the field effect transistor M1 is responsive to the control signal from the error amplifier 12 for maintaining the output voltage produced across the output terminals 3 and 4 at the correct regulated voltage value.
Such linear voltage regulators as the prior art linear voltage regulator of FIG. 1 have many advantages, one of which is that they tolerate a relatively wide variation in the voltage of the unregulated DC voltage supply between the minimum voltage to which the unregulated voltage supply may drop and the maximum voltage to which the unregulated voltage supply may rise. Additionally, such linear voltage regulators operate with a relatively small voltage difference between the unregulated voltage supply and the regulated output voltage. In other words, the dropout voltage which is the voltage of the unregulated voltage supply at which the voltage regulator ceases to produce the regulated voltage is relatively low, and in general is of value just above the regulated voltage value.
However, a disadvantage of such linear voltage regulators is that since the load current Il is drawn through the pass element 7, the power dissipated by the pass element 7 is equal to the product of the load current Il by the voltage drop across the pass element. The voltage drop across the pass element is equal to the value of the voltage of the unregulated voltage supply less the value of the regulated output voltage. Thus, as the voltage of the unregulated voltage supply increases, the power dissipated by the pass element 7 also increases. Since the power dissipated by the pass element 7 is dissipated as heat, the heat produced by the pass element can be relatively high, and in particular, can be relatively high at the higher values of the voltage of the unregulated voltage supply. This is undesirable, and is particularly undesirable when the linear voltage regulator is implemented as an integrated circuit on a die due to the difficulty in dissipating heat from dies. The problem of heat dissipation is further aggravated when the load to which the regulated output voltage is being supplied is implemented as an integrated circuit on the same die or package as that on which the linear voltage regulator is formed.
Efforts have been made to address the problem of heat dissipation in prior art linear voltage regulators of the type illustrated in FIG. 1. A typical prior art linear voltage regulator which reduces heat dissipation on a die is illustrated in FIG. 2. In the linear voltage regulator of FIG. 2 a voltage dropping resistor Rext is provided in series with the pass element 7, but is located externally of the die. Thus, the external voltage dropping resistor Rext drops some of the voltage between the unregulated voltage supply and the regulated output voltage while the remainder of the voltage between the unregulated voltage supply and the regulated output voltage is dropped across the pass element. This in turn splits the power dissipated by the voltage regulator between the power dissipated by the pass element 7 in the die and the power dissipated by the external resistor Rext externally of the die. However, a problem with this voltage regulator is that the dropout voltage, in other words, the value of the unregulated voltage supply at which the voltage regulator ceases to produce the regulated output voltage is increased by the voltage drop across the external resistor Rext.
Accordingly, the external resistor Rext should be selected to have a maximum resistance value sufficiently low that, at the minimum value of the unregulated voltage supply and when the current drawn by the load is a maximum, the voltage dropped across the external resistor Rext is such that the voltage regulator continues to produce the regulated output voltage. This, however, imposes a limitation on the size of the external resistor Rext, and in turn the amount of heat which can be dissipated by the external resistor Rext.
A computer simulation of the voltage regulator of FIG. 2 was carried out. The results of the computer simulation are illustrated by the graphs of FIG. 3 which are described below. In the computer simulation the voltage regulator was configured to operate with an unregulated voltage supply which varies between a minimum voltage value of 11 volts and a maximum voltage value of 25 volts, and to produce a regulated output voltage of 5 volts with a maximum load current of 50 mA. The resistance value of the external resistor Rext was selected to be sufficiently low that the voltage drop across the external resistor Rext was less than 6 volts. Otherwise, during periods when the unregulated voltage supply remained at its minimum voltage value, the voltage available to the pass element 7 would be insufficient for the voltage regulator to produce the regulated output voltage. In this case the resistance value of the external resistance was selected to be 120 ohms. Therefore, in this particular case when the unregulated supply voltage reached its maximum value of 25 volts, the voltage dropped across the external resistor Rext was still less than 6 volts, thus leaving a voltage of 14 volts to be dropped across the pass element 7. This resulted in a relatively high power dissipation by the pass element 7 in the form of heat, particularly at the relatively higher values of the unregulated input voltage.
Referring now in particular to FIG. 3, FIG. 3 illustrates three graphs which represent power dissipated by the voltage regulator of FIG. 2 plotted against the unregulated input voltage obtained from the computer simulation. In FIG. 3 power is plotted in watts on the vertical Y-axis, and the voltage of the unregulated input voltage is plotted in volts on the horizontal X-axis. Graph X represents a plot of the total power dissipated by the external resistor Rext and the pass element 7 of the voltage regulator of FIG. 2 as the unregulated input voltage varies between the minimum value of 11 volts and the maximum value of 25 volts. Graph Y represents the power dissipated internally in the voltage regulating circuit of FIG. 2 by the pass element 7 as the unregulated input voltage varies from the minimum value of 11 volts to the maximum value of 25 volts. Graph Z represents the power dissipated by the external resistor Rext as the unregulated input voltage varies from the minimum value of 11 volts to the maximum value of 25 volts. As can be seen from graph Y, the power dissipated by the pass element 7 increases from zero watts to approximately 0.7 watts as the unregulated input voltage varies from 11 volts to 25 volts. However, from graph Z it can be seen that the power dissipated by the external resistor Rext remains constant at approximately 0.3 watts as the unregulated input voltage varies between 11 volts and 25 volts. Thus, the total power dissipated by the external resistor Rext and the pass element 7, as can be seen from graph X of FIG. 3, varies from approximately 0.3 watts to 1 watt. Accordingly, at the maximum value of the unregulated input voltage of 25 volts, the pass element dissipates approximately 0.7 watts, and since all the power dissipated by the pass element 7 is dissipated in the form of heat, the heat dissipated internally in the voltage regulating circuit of FIG. 2 by the pass element 7 is approximately 0.7 watts. However, at the maximum value of 25 volts of the unregulated input voltage, the external resistor Rext is still only dissipating the same amount of power, namely, approximately 0.3 watts, as it dissipates when the unregulated input voltage is at its minimum value of 11 volts when virtually no heat is being dissipated by the pass element 7. This is clearly undesirable.
Thus, while the provision of the external resistor Rext assists to some extent in externally dissipating power and in turn heat produced by the linear voltage regulator of FIG. 2, its benefit is limited, particularly in cases where the unregulated voltage supply varies widely between an upper maximum voltage value and a lower minimum voltage value.
There is therefore a need for a voltage regulating circuit which produces a regulated DC output voltage from an unregulated DC voltage supply, which addresses the problem of power dissipation by prior art voltage regulators.
The present disclosure provides such a DC voltage regulating circuit, and the disclosure is also directed towards providing a method for producing a regulated DC output voltage from an unregulated DC voltage supply which addresses the problem of power dissipation of known voltage regulators.